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SOI Structure pressure transducer formed by oxidized porous silicon
840
Sensors and Actuators, A21-A23 (1990) 840-843
SO1 Structure Pressure Transducer Formed by Oxidized Porous Silicon GANMING ZHAO, YIPING H U A N G and MINHANG BAO
Department of Electronic Engineering, Fudan University, Shanghai (China)
Abstract A new type of SO1 structure pressure transducer formed by oxidized porous silicon with a buried harrier-layer has been developed. By using unique SOI structure, single crystal silicon piezoresistances are insulated from monocrystalline silicon substrate by dioxide. The breakdown voltage between two separate piezoresistors is more than 200Vd.c. The dimensions of piezoresistors are 12 #m × 68 #m, and the dimensions of the silicon rectangular diaphragm are 0.86 m m x 1.18 nun after etching from the back. For a 50/~m thick diaphragm, a sensitivity of 9.5 mV/5 V kg/cm 2 has been obtained. The dependence of sensitivity on temperature was measured from 20 to 350 °C. The experimental results show that this device can be ufflized at significantly higher operating temperatures (up to 350 °C). The average temperature coefficient of sensitivity is about - 9 x 10-4/°C. The design, fabrication process and the characteristics of the transducer are also discussed.
Introduction In recent years, silicon pressure transducers have been developing continuously [ 1]. A variety of pressure transducers [2, 3] fabricated on single crystal substrate show very good performances. However, an obvious disadvantage is that they can only work at temperatures under 120 °C. This limitation is caused mainly by the p - n junction isolation. In order to overcome this shortcoming while maintaining the good performances of the single crystal silicon pressure transducer, some pressure transducers with SOI (silicon-on-insulator) structures have been reported in recent years, focusing on SOS structure pressure transducers [4] and polysilicon pressure transducers [5]. Although both of them can work in high operating temperatures, SOS structure pressure transducers have problems in hetero-structure processing as well as batch production, and polysilicon pressure transducers have a much lower gauge factor. 0924-4247/90/$3.50
Reported in this paper is a new type of pressure transducer used in high temperatures with high sensitivity. This transducer is fabricated by a unique SOI structure by using oxidized porous silicon. The experimental results indicate that this SOI structure pressure transducer has the advantages of a simple formation process, high sensitivity, a wide operating temperature range, good stability, and can be batch produced.
Formation of SO1 Structme Recently the single crystalline silicon-on-insulator structure (SOI) has attracted a special interest for its perfect isolation. Several approaches to obtain SOI structures are realized by SOS (siliconon-sapphire), SIMOX (separation by implanted oxygen), laser annealing and zone melting, etc. Among them, oxidation of porous silicon is one of the very good prospects in the field of integrated circuits and solid-state transducers. Using this SOI technology, single crystalline silicon thin film of good quality surrounded by dioxide can be obtained. In 1984, Imai [6] reported the FIPOS (full insulation by porous oxidized silicon) technology and its application to LSIs. This technology suffers from the disadvantages of the thicker layer of porous silicon and larger stress. In 1986, Benjaman et ai. [7] described a process for the formation of a silicon-on-insulator structure by selective anodization of a buried p-type layer to form porous silicon which is then oxidized. The use of proton implantation makes the process more complicated than others. In the present work we used an improved technology of porous oxidized silicon to obtain a SOI structure which is then used in the fabrication of the SOI structure pressure transducer. The material used in this work is the p-type, (100) oriented Si wafer with resistivity of 5 6 t~ cm. The key processing steps for the formation of the SOI structure are described below and shown in Fig. 1. (a) Patterns on a SiO2 film about 2000 A thick are formed as N + buried layer diffusion masks on © Elsevier Sequoia/Printed in The Netherlands
841 I I
n*
I
[
n+
p(lOO)-Si
(a)
Boron ,I. ! ! I
n+
Boron ~n • I
I
n+
I
~ .J
I
n-epi-
~ p-epi
p-Si
(b) n-resistor
!{~
n +
~x...:,!,
porous silicon
n +
Fig. 2. Cross-sectional view o f the SO1 structure.
p-Si
(c)
n-resistor 1~/73;./~/~/7~73////I H ~,/,, ,J__ oxldixed I~'A n+ Y/hi n+ IrA porous [.;'1~;;. "J silicon
(d) I
p-si
|
Fig. 1. Key process steps for a new type o f SOl structure.
a p-type silicon substrate. Then antimony is diffused into the silicon substrate to form a N + barrier layer. The pressure of this layer forces the current of anodic reaction to flow laterally rather than down into the substrate. (b) A p-type 2 #m thick epitaxial layer with resistivity of 1-1.5 Q cm is deposited on the substrate. A 2 #m n-type epitaxial layer with resistivity of 0 . 8 D c m is then grown on the p-type epitaxy layer. This upper layer is to be used as ma erial for the piezoresistive SOI structure pressure transducer. Then, boron diffusion is performed to form isolated n-resistors. (c) Anodization is performed in a cell with two chambers in which contact is made to the surfaces of the wafer with hydrofluoric acid. The anodization is carried out in the dark. In this anodic reaction, only p-type silicon around an n-island is changed to porous silicon. The concentration of hydrofluoric acid used is 42% and the current density is 70 mA/cm 2. The anodic reaction takes 10 rain. (d) The porous silicon is thermally oxidized at 700 °C for 2 h in wet O2 ambient. Since the oxidation rate of porous silicon is 10-20 times greater than that of single crystal silicon, fully isolated n-type single crystalline silicon islands can be obtained. By means of the above technology, the n-type single crystal resistors are surrounded by dioxide, as shown in Fig. 2. The measured breakdown
voltage between the two separated resistors is more than 200 V d.c. The width of the isolated resistor can be larger than 30/~m.
SO1 Structure Pressure Tramsducer
By using the unique technology mentioned above, we have obtained the single crystalline silicon-on-insulator structure and used it for piezoresistive pressure transducers. In designing this transducer, the single crystalline silicon islands of SOI structure are used as piezoresistors. They are insulated from each other and from the silicon substrate by silicon dioxide. Four piezoresistors on every chip are located at the center and the edge of a rectangular diaphragm. After the formation of the SOI structure, boron ion implantation with an energy of 100 keV and dosage of 1 x 10~4c m - 2 was performed to change n-type resistors into p-type piezoresistors. Then after a few more steps of the IC process, such as photolithography, metallization and anisotropic etching, the pressure transducer with SOI structure is complete. The photomicrograph of the transducer is shown in Fig. 3. The dimensions of the resistors are 12/tm x 68/~m, and the dimensions of the silicon diaphragm are 0.8 mm x 1.18 mm after etching from the back. The sensitivity of this SOI structure pressure transducer is
AR ~"(r~- TJ R
2
(l)
where ~ is the piezoresistive coefficient for a p-type single crystal silicon in the crystallographic coordinate system. T~ and Tt are the longitudinal and transverse stress, respectively. The expressions for T~ and Tt in a rectangular diaphragm system are well known [8].
842
~T ~
1.25 1O0
1 !
",-
++/++++++
200 I
300 ! +++.++
+
.I
" T(°C)
~b.7s, 0.5(
Fig. 5. Experimental results of the dependence of sensitivity on temperature.
Fig. 3. Piezoresistive pressure transducer with SO1 structure.
Experiments and Results Figure 4 shows the output voltage of a piezoresistive bridge in a 50/~m thick rectangular diaphragm as a function of pressure. The excitation voltage is 5 V d.c. This measurement indicates that the output voltage of this SOI structure pressure transducer is linear to the supplied pressure. The sensitivity of the transducer is 9.5 mV/5 V kg/cm 2. This sensitivity is very close to that of the single crystal pressure transducer and agrees well with the value obtained from eqn. (1). The experiments demonstrate that this SOI structure pressure transducer maintains its sensitivity as high as that of the single crystal transducer. The dependence of sensitivity on temperature was measured from 20 to 350 °C. The result of this experiment is shown in Fig. 5. The sensitivity decreases by about 30% in the full temperature range. Therefore, the average temperature coefficient of sensitivity (TCS) is about
- 9 x 10-4/°C. This result shows that this SOI structure sensing element can work at high operating temperatures up to 350 °C with the proper transducer package. In addition, the anodizing condition can be adjusted so that the porous silicon has a density of 45% of that of bulk silicon. Therefore, the volume increase in oxidation can be compensated for and stress caused by oxidation is minimized.
Conelmions In order to fabricate SOI structure pressure transducers which work at high temperatures, porous oxidized silicon technology with a buried barrier-layer has been proposed. The output characteristics of this SOI structure pressure transducer show high sensitivity like the single crystal silicon transducer, high operating temperatures up to 350 °C and good stability. The SOI structure pressure transducer gives encouraging results. It is expected that the pressure transducers formed by oxidized porous silicon will find wide applications when a high working temperature is required.
Acknowledgments
10
This project is partly supported by the Joint Laboratories of Transducing Technology of China. The authors are grateful to Miss Sheng Ja-ying and Mrs Dou Feng-chung for assistance in the experiments, and all members of the Microelectronics Research Laboratory for their continuous support of this work.
8
~6
>
I 0.2
I I 0.4 0.6 P (kg/cm 2}
I 0.8
I 1.0
Fig. 4. Output voltage of a piezoresistive bridge as a function of applied pressure.
References 1 Minhang Bao, Yan Wang and Weijia Qi, New development of design method of piezore~ttive pressure sensor, Tech.
Digest, Int. Conf. Solid-State Sensors and Actuators, Tokyo, Japan, 1987, pp. 299-304.
843 2 S. K. Clark and K. D. Wise, Pressure sensitivity in anisotropically etched thin diaphragm pressure sensors, IEEE Trans. Electron Devices, ED-26 (1979) 1887-1896. 3 Y. Kanda and A. Yasukawa, Hall-effect devices as strain and pressure sensors, Sensors and Actuators, 2 (1982) 283-296. 4 Q. G. Chen, J. H. Zhang and Z. J. Teng, Silicon-on-sapphire pressure sensor, Int. Conf. Solid-State Sensors and Actuators, Tokyo, Japan, 1987, pp. 320-323. 5 P. J. French and A. G. R. Evans, Polysificon strain sensors using shear piezoresistance, Sensors and Actuators, 15 (1988) 257 -272.
6 K. Imai and H. Unno, FIPOS (full isolation by porous oxidized silicon) technology and its application to LSIs, 1EEE Trans. Electron Devices, ED-31 (1984) 297302. 7 J. D. lkniaman, J. M. Keen, A. G. Cullis, B. Innes and N. G. Chew, Large area, uniform silicon-on-insulator using a buried layer of oxidized porous sificon, Appi. Phys. Lett., 49 (1986) 716-718. 8 Minhang Bao and Yan Wang, Analysis and design of a four-terminal silicon pressure sensor at the centre of a diaphragm, Sensors and Actuators, 12 (1987) 49-56.
Sensors and Actuators, A21-A23 (1990) 840-843
SO1 Structure Pressure Transducer Formed by Oxidized Porous Silicon GANMING ZHAO, YIPING H U A N G and MINHANG BAO
Department of Electronic Engineering, Fudan University, Shanghai (China)
Abstract A new type of SO1 structure pressure transducer formed by oxidized porous silicon with a buried harrier-layer has been developed. By using unique SOI structure, single crystal silicon piezoresistances are insulated from monocrystalline silicon substrate by dioxide. The breakdown voltage between two separate piezoresistors is more than 200Vd.c. The dimensions of piezoresistors are 12 #m × 68 #m, and the dimensions of the silicon rectangular diaphragm are 0.86 m m x 1.18 nun after etching from the back. For a 50/~m thick diaphragm, a sensitivity of 9.5 mV/5 V kg/cm 2 has been obtained. The dependence of sensitivity on temperature was measured from 20 to 350 °C. The experimental results show that this device can be ufflized at significantly higher operating temperatures (up to 350 °C). The average temperature coefficient of sensitivity is about - 9 x 10-4/°C. The design, fabrication process and the characteristics of the transducer are also discussed.
Introduction In recent years, silicon pressure transducers have been developing continuously [ 1]. A variety of pressure transducers [2, 3] fabricated on single crystal substrate show very good performances. However, an obvious disadvantage is that they can only work at temperatures under 120 °C. This limitation is caused mainly by the p - n junction isolation. In order to overcome this shortcoming while maintaining the good performances of the single crystal silicon pressure transducer, some pressure transducers with SOI (silicon-on-insulator) structures have been reported in recent years, focusing on SOS structure pressure transducers [4] and polysilicon pressure transducers [5]. Although both of them can work in high operating temperatures, SOS structure pressure transducers have problems in hetero-structure processing as well as batch production, and polysilicon pressure transducers have a much lower gauge factor. 0924-4247/90/$3.50
Reported in this paper is a new type of pressure transducer used in high temperatures with high sensitivity. This transducer is fabricated by a unique SOI structure by using oxidized porous silicon. The experimental results indicate that this SOI structure pressure transducer has the advantages of a simple formation process, high sensitivity, a wide operating temperature range, good stability, and can be batch produced.
Formation of SO1 Structme Recently the single crystalline silicon-on-insulator structure (SOI) has attracted a special interest for its perfect isolation. Several approaches to obtain SOI structures are realized by SOS (siliconon-sapphire), SIMOX (separation by implanted oxygen), laser annealing and zone melting, etc. Among them, oxidation of porous silicon is one of the very good prospects in the field of integrated circuits and solid-state transducers. Using this SOI technology, single crystalline silicon thin film of good quality surrounded by dioxide can be obtained. In 1984, Imai [6] reported the FIPOS (full insulation by porous oxidized silicon) technology and its application to LSIs. This technology suffers from the disadvantages of the thicker layer of porous silicon and larger stress. In 1986, Benjaman et ai. [7] described a process for the formation of a silicon-on-insulator structure by selective anodization of a buried p-type layer to form porous silicon which is then oxidized. The use of proton implantation makes the process more complicated than others. In the present work we used an improved technology of porous oxidized silicon to obtain a SOI structure which is then used in the fabrication of the SOI structure pressure transducer. The material used in this work is the p-type, (100) oriented Si wafer with resistivity of 5 6 t~ cm. The key processing steps for the formation of the SOI structure are described below and shown in Fig. 1. (a) Patterns on a SiO2 film about 2000 A thick are formed as N + buried layer diffusion masks on © Elsevier Sequoia/Printed in The Netherlands
841 I I
n*
I
[
n+
p(lOO)-Si
(a)
Boron ,I. ! ! I
n+
Boron ~n • I
I
n+
I
~ .J
I
n-epi-
~ p-epi
p-Si
(b) n-resistor
!{~
n +
~x...:,!,
porous silicon
n +
Fig. 2. Cross-sectional view o f the SO1 structure.
p-Si
(c)
n-resistor 1~/73;./~/~/7~73////I H ~,/,, ,J__ oxldixed I~'A n+ Y/hi n+ IrA porous [.;'1~;;. "J silicon
(d) I
p-si
|
Fig. 1. Key process steps for a new type o f SOl structure.
a p-type silicon substrate. Then antimony is diffused into the silicon substrate to form a N + barrier layer. The pressure of this layer forces the current of anodic reaction to flow laterally rather than down into the substrate. (b) A p-type 2 #m thick epitaxial layer with resistivity of 1-1.5 Q cm is deposited on the substrate. A 2 #m n-type epitaxial layer with resistivity of 0 . 8 D c m is then grown on the p-type epitaxy layer. This upper layer is to be used as ma erial for the piezoresistive SOI structure pressure transducer. Then, boron diffusion is performed to form isolated n-resistors. (c) Anodization is performed in a cell with two chambers in which contact is made to the surfaces of the wafer with hydrofluoric acid. The anodization is carried out in the dark. In this anodic reaction, only p-type silicon around an n-island is changed to porous silicon. The concentration of hydrofluoric acid used is 42% and the current density is 70 mA/cm 2. The anodic reaction takes 10 rain. (d) The porous silicon is thermally oxidized at 700 °C for 2 h in wet O2 ambient. Since the oxidation rate of porous silicon is 10-20 times greater than that of single crystal silicon, fully isolated n-type single crystalline silicon islands can be obtained. By means of the above technology, the n-type single crystal resistors are surrounded by dioxide, as shown in Fig. 2. The measured breakdown
voltage between the two separated resistors is more than 200 V d.c. The width of the isolated resistor can be larger than 30/~m.
SO1 Structure Pressure Tramsducer
By using the unique technology mentioned above, we have obtained the single crystalline silicon-on-insulator structure and used it for piezoresistive pressure transducers. In designing this transducer, the single crystalline silicon islands of SOI structure are used as piezoresistors. They are insulated from each other and from the silicon substrate by silicon dioxide. Four piezoresistors on every chip are located at the center and the edge of a rectangular diaphragm. After the formation of the SOI structure, boron ion implantation with an energy of 100 keV and dosage of 1 x 10~4c m - 2 was performed to change n-type resistors into p-type piezoresistors. Then after a few more steps of the IC process, such as photolithography, metallization and anisotropic etching, the pressure transducer with SOI structure is complete. The photomicrograph of the transducer is shown in Fig. 3. The dimensions of the resistors are 12/tm x 68/~m, and the dimensions of the silicon diaphragm are 0.8 mm x 1.18 mm after etching from the back. The sensitivity of this SOI structure pressure transducer is
AR ~"(r~- TJ R
2
(l)
where ~ is the piezoresistive coefficient for a p-type single crystal silicon in the crystallographic coordinate system. T~ and Tt are the longitudinal and transverse stress, respectively. The expressions for T~ and Tt in a rectangular diaphragm system are well known [8].
842
~T ~
1.25 1O0
1 !
",-
++/++++++
200 I
300 ! +++.++
+
.I
" T(°C)
~b.7s, 0.5(
Fig. 5. Experimental results of the dependence of sensitivity on temperature.
Fig. 3. Piezoresistive pressure transducer with SO1 structure.
Experiments and Results Figure 4 shows the output voltage of a piezoresistive bridge in a 50/~m thick rectangular diaphragm as a function of pressure. The excitation voltage is 5 V d.c. This measurement indicates that the output voltage of this SOI structure pressure transducer is linear to the supplied pressure. The sensitivity of the transducer is 9.5 mV/5 V kg/cm 2. This sensitivity is very close to that of the single crystal pressure transducer and agrees well with the value obtained from eqn. (1). The experiments demonstrate that this SOI structure pressure transducer maintains its sensitivity as high as that of the single crystal transducer. The dependence of sensitivity on temperature was measured from 20 to 350 °C. The result of this experiment is shown in Fig. 5. The sensitivity decreases by about 30% in the full temperature range. Therefore, the average temperature coefficient of sensitivity (TCS) is about
- 9 x 10-4/°C. This result shows that this SOI structure sensing element can work at high operating temperatures up to 350 °C with the proper transducer package. In addition, the anodizing condition can be adjusted so that the porous silicon has a density of 45% of that of bulk silicon. Therefore, the volume increase in oxidation can be compensated for and stress caused by oxidation is minimized.
Conelmions In order to fabricate SOI structure pressure transducers which work at high temperatures, porous oxidized silicon technology with a buried barrier-layer has been proposed. The output characteristics of this SOI structure pressure transducer show high sensitivity like the single crystal silicon transducer, high operating temperatures up to 350 °C and good stability. The SOI structure pressure transducer gives encouraging results. It is expected that the pressure transducers formed by oxidized porous silicon will find wide applications when a high working temperature is required.
Acknowledgments
10
This project is partly supported by the Joint Laboratories of Transducing Technology of China. The authors are grateful to Miss Sheng Ja-ying and Mrs Dou Feng-chung for assistance in the experiments, and all members of the Microelectronics Research Laboratory for their continuous support of this work.
8
~6
>
I 0.2
I I 0.4 0.6 P (kg/cm 2}
I 0.8
I 1.0
Fig. 4. Output voltage of a piezoresistive bridge as a function of applied pressure.
References 1 Minhang Bao, Yan Wang and Weijia Qi, New development of design method of piezore~ttive pressure sensor, Tech.
Digest, Int. Conf. Solid-State Sensors and Actuators, Tokyo, Japan, 1987, pp. 299-304.
843 2 S. K. Clark and K. D. Wise, Pressure sensitivity in anisotropically etched thin diaphragm pressure sensors, IEEE Trans. Electron Devices, ED-26 (1979) 1887-1896. 3 Y. Kanda and A. Yasukawa, Hall-effect devices as strain and pressure sensors, Sensors and Actuators, 2 (1982) 283-296. 4 Q. G. Chen, J. H. Zhang and Z. J. Teng, Silicon-on-sapphire pressure sensor, Int. Conf. Solid-State Sensors and Actuators, Tokyo, Japan, 1987, pp. 320-323. 5 P. J. French and A. G. R. Evans, Polysificon strain sensors using shear piezoresistance, Sensors and Actuators, 15 (1988) 257 -272.
6 K. Imai and H. Unno, FIPOS (full isolation by porous oxidized silicon) technology and its application to LSIs, 1EEE Trans. Electron Devices, ED-31 (1984) 297302. 7 J. D. lkniaman, J. M. Keen, A. G. Cullis, B. Innes and N. G. Chew, Large area, uniform silicon-on-insulator using a buried layer of oxidized porous sificon, Appi. Phys. Lett., 49 (1986) 716-718. 8 Minhang Bao and Yan Wang, Analysis and design of a four-terminal silicon pressure sensor at the centre of a diaphragm, Sensors and Actuators, 12 (1987) 49-56.